New regulations have been designed to protect the atmospheric ozone layer from possible damage by fully halogenated chlorofluorocarbons, also known as CFC's. Pentafluoroethane (CHF2--CF3 or HFC-125) is a useful non-chlorine containing fluorocarbon that is especially desirable for use as a refrigerant, blowing agent, propellant, fire extinguishing agent, sterilant carrier gas, among other uses.
Pentafluoroethane may be prepared by chlorofluorinating perchloroethylcne (perclene or CC12.dbd.CC12) to produce a mixture comprising trichlorotrifluoroethane (CF2Cl--CFC12 or CFC-113), dichlorotetrafluoroethane (CF2Cl--CF2Cl or CFC-114) and dichlorotrifluoroethane (CHCl2--CF3 or HCFC-123), thereafter removing trichlorotrifluoroethane, and; fluorinating the remaining mixture to produce a mixture comprising pentafluoroethane (HFC-125), chloropentafluoroethane (CF3--CF2Cl or CFC-115), and smaller amounts of other fluorinated compounds, e.g., hexafluoroethane (CF3--CF3 or FC-116). Such a chlorofluorinating method is described in U.S. Pat. No. 3,755,477. Various other methods for making pentafluoroethane together with chloropentafluoroethane are known and described, for example, in U.S. Pat. Nos. 3,258,500, 5,334,787 and 5,399,549, Japanese Patent Application Publication Nos. JP 03/099026, JP 04/029941, European Patent Application Publication No 0 506 525, and World Intellectual Property Organization Publication No. WO 91/05752.
The presence of chloropentafluoroethane (CFC-115) in HFC-125 is considered objectionable, because CFC-115 is a chlorofluorocarbon (CFC) and, therefore, believed capable of harming the ozone layer. However, CFC-115 removal is difficult because the boiling points of pentafluoroethane and chloropentafluoroethane are relatively close, i.e., about -48.5 degrees C and -38.7 degrees C., respectively, and under certain conditions form a known azeotropic or azeotrope-like composition. The above boiling points and azeotropic or azeotrope-like compositions indicate that it would be extremely difficult if not impossible to recover substantially pure pentafluoroethane from such a mixture by conventional distillation. By "conventional distillation" is meant that the relative volatility only of the components of the mixture to be separated is being used to separate them.
The difficulty of separating pentafluoroethane from chloropentafluoroethane is well-recognized, and a number of approaches have been proposed for separating these compounds. U.S. Pat. No. 5,346,595 discloses a method of separating chloropentafluoroethane from pentafluoroethane by a series of distillations performed at different pressures to take advantage of small changes of azeotropic composition with pressure. This process requires multiple distillations and is energy inefficient. U.S. Pat. No. 5,087,329 discloses a method of separating chloropentafluoroethane from pentafluoroethane by extractive distillation with a fluorocarbon extractive agent.
WO 94/02439 discloses a method of removing chlorine-containing impurities from pentafluoroethane by selectively reacting the chlorocarbon with hydrogen. EP 612709 discloses a similar method for removing chloropentafluoroethane from pentafluoroethane by selectively reacting the chlorocarbon with fluorine. Such processes are expensive to perform and may result in some loss of pentafluoroethane by reaction.
WO 94/22793 discloses a method of separating chloropentafluoroethane from pentafluoroethane by contacting the mixture with certain molecular sieves or activated carbons. This process requires the periodic desorption of chloropentafluoroethane from the adsorbent, thus requiring multiple units for continuous operation.
Hydrofluoric acid (HF) is normally a common component of the various fluorination reactions that may be used to produce HFC-125, and it is similarly extremely difficult to remove completely from the HFC-125. Hydrofluoric acid and HFC-125 form an azeotrope that make complete separation of the hydrofluoric acid from the HFC-125 by conventional distillation virtually impossible. The formation of an azeotropic or azeotrope-like composition between HFC-125 and hydrofluoric acid is disclosed in WO96/09271. The presence of HF in HFC-125 is considered objectionable. One method for removing HF from HFC-125 comprises scrubbing the HFC-125 with water. Such a method, however, reduces the value of the scrubbed HF as a raw material in that such cannot be recycled back to the HFC-125 reactor, requires the water wash to be subsequently treated prior to disposition, and; introduces water into the HFC-125 product.
Difluoromethane (CH2F2 or HFC-32) is another desirable non-chlorine containing fluorocarbon that is also valuable as a refrigerant and among other uses. HFC-32 can be formed, for example, by the catalytic fluorination of dichloromethane (HCC-30 or methylene chloride) with HF. HFC-32 is known to form an azeotropic or azeotrope-like composition with CFC-115, and the azeotrope's existence has been disclosed in "Fluorocarbon Refrigerants Handbook", by R. C. Downing, Prentice Hall,1988; U.S. Pat. No. 3,470,101 to Broadley; and "Pressure-Volume-Temperature Behavior of a Mixture of Difluoromethane and Pentafluoroethane" by Mears et al., Journal of Chemical and Engineering Data, Vol. 13, No. 3, July 1968.
The disclosure of the previously identified patents and publications is hereby incorporated by reference.